That's unfortunate as with bugs like these, if they persist, the only way to fix them would be to contact support, but that is practically guaranteed to get you banned if you've been using the modded apk.
The only other thing that I could recommend is to move to another distant area (Try the Ministry or an area off Hogwarts grounds), save, and then use the Clear Cache option. You can then try to use the complete/collect button again and see if you are able to interact with Mcgonagall. Otherwise, if that doesn't work, you can try waiting for a least 24 hours and then try again. I know that some parts of the game (such as dueling) have failsafes built in so that they reset after 24 hours in case you become stuck.